This invention relates to devices for measuring displacement, and more particularly to a wireless device that can be implanted between two adjacent objects and used to measure changes in their separation distance.
Displacement and proximity sensors play large roles in the automotive, aerospace, food, beverage, metal, and computer industries. The increase in automation has vastly increased the demand for such sensors. This demand is due to the replacement of outdated plant equipment and the overall increase in factory automation.
Of the sensors in the proximity and displacement sensor market, inductive (magnetic) and photoelectric sensors are probably the most popular. Other types of displacement sensors are capacitive sensors, ultrasonic sensors, potentiometric sensors, laser sensors, and ultrasonic sensors.
Magnetic displacement sensors include LVDT (linear variable differential transform) sensors, hall effect sensors, and magnetostrictive sensors. LVDT sensors use three coils, a primary coil and two secondary coils. The secondary coils are connected to establish a null position. A magnetic core inside the coil winding assembly provides a magnetic flux. When the core is displaced from the null position, an electromagnetic imbalance occurs. Hall effect sensors are based on a voltage that is generated in one direction when a current and a magnetic field pass through semiconductor material in the other two perpendicular directions.
Variations of magnetic and inductive sensors have been developed with one or two coils. A disadvantage of many magnetic and inductive designs is the need for an electrical connection to the sensor.
One aspect of the invention is a sensor/interrogator system for measuring displacement between two adjacent objects. The sensor has a magnetic rod, a sensor coil, and a capacitor attached to the sensor coil so as to form a tuned circuit. A first end of the rod is insertable into a first end of the coil and moveable along the axis of the coil. The rod has an end mount at its second end, as does the coil, which permits the sensor to be attached between the two objects. When the objects move, the rod moves along the coil. The interrogator having at least one interrogator coil, transmit circuitry for delivering to the sensor coil an excitation signal through a range of frequencies, and receive circuitry for receiving a response signal from the sensor coil. The change in frequency of the response signal is related to the amount of motion of the rod inside the coil.
For orthopedic applications, an advantage of the invention is that it provides a non-invasive system that incorporates an implantable passive sensor and an external interrogating device. The system is especially useful to diagnose spinal fusion postoperatively, by measuring the changes in separation of the vertebrae. The sensor response can be correlated to the relative motion of the vertebrae. The system can also be used for diagnosing other types of bone fusion, such as motion between an orthopedic implant and the surrounding bone. Small motions in this case, indicate implant loosening. The system can also measure motion between two bone segments of a fracture. Small motions in this case, indicate non-fusion of the fracture.
For spinal fusion applications, when a patient postoperatively complains of pain, the physician needs to determine whether the pain is the same as the preoperative pain or if it is from a different source. The sensor/interrogator system may be used to diagnose whether the spine has fused (a new source of pain must be the cause) or not (the same area may be causing the pain). This determination will affect the patient""s treatment. In addition, as the patient is monitored postoperatively, the physician can use the information from the system to guide the patient""s rehabilitation program, allowing a faster recovery time and reduced healthcare costs. In the past, methods to diagnose spinal fusion have used radiographic tools. In contrast, the system described herein does not need radiography, and allows the physician to diagnose spinal fusion in his or her office.